1. Field of the Invention
This invention relates to power factor correction and more particularly to a system and method for correcting the electrical power factor of an accessory to a main equipment by utilizing the power factor corrector of the main equipment.
2. Description of the Related Art
In many high current commercial systems, a large part of the current drain is due to poor power factor (i.e. a substantial out of phase relationship between the voltage and current). This can cause a serious overloading of the electrical power system and may require special corrective action by the power company. In addition, a substantial surcharge is charged by most power companies to commercial users whose equipment presents a poor power factor to the power lines.
The widespread use of high frequency switching regulated power supplies in various type of commercial equipment presents a particular power factor problem in that such systems employ large energy storage, electrolytic capacitors at the output of the rectifier and the input to the DC-DC converter used to convert the rectified supply voltage to low voltage DC. Such capacitive loading results in a haversine pulsating current at the line frequency which tends to occur near the peak of the line voltage. This results in high RMS input currents, with substantial distortion and a low power factor on the input line. To alleviate this problem, new types of switching power supplies have been designed which incorporate active electronic power factor correction circuitry which effectively reduces the RMS current drain and the harmonic current distortion. Such power supplies are commercially available from various companies including Modular Devices Incorporated, Torrance, Calif. (models PS 2129, PS 2133 and PS2118). Power supplies of this type are described in various published articles including: "Correct Power Factor in Switched Mode Power Supplies" by Bradford Hall, published in Electronic Design magazine, Oct. 27, 1988 and "PWM Controlled Chip Fixes Power Factor" by Frank Goodenough, Electronic Design magazine, June 1989.
While a power factor corrected power supply is generally provided with a larger system, it is often not present in accessory equipment used with such a main equipment. Thus, while the main system has adequate power factor correction, the overall power factor presented to the line, in view of the load of the accessories may be unacceptable. This problem could, of course be solved either by providing a separate independent power factor corrector in the supplies for the accessories or by running such accessories from the high voltage DC in the main power supply. The first of these options is undesirable because it requires the modification of the power supply furnished with the accessory at substantial expense. The second option has the disadvantage of placing an additional load on the main power supply which could cause overloading.